Commercial MgB2 Research Articles

Fabrication of Ru loaded MgB2 with guar gum hybrid for photocatalytic degradation of crystal violet

Today, dyes/pigment-based materials are confronting a serious issue in harming marine ecology. Annihilate these serious water pollutants using photoactive 2D nanohybrid catalysts showed promising comparativeness over available photocatalysts. In the present work, a facile route to decorate Ruthenium (Ru) on 2D MgB2 flower-like nanostructures was developed via ecofriendly guar gum biopolymer substantial template (MgB2/GG@Ru NFS) and its photocatalytic performance was reported. Synthesis of MgB2@Ru, MgB2/GG@Ru NFS and commercial MgB2, was studied by FTIR, XRD, FE-SEM, EDX, AFM, TEM, UV–vis spectra, and XPS analysis. From the results, the MgB2/GG@Ru NFS exhibited a superior photocatalytic performance (99.7 %) than its precursors MgB2@Ru (79.7 %), and MgB2 (53.7 %), with the degradation efficiency of the crystal violet (CV) within 100 min under visible light irradiation. The proposed photo-catalyst MgB2/GG@Ru NFS showed negligible loss of photocatalytic activity even after five successive cycles, revealing its reusability and enhanced stability due to the network structure. The photocatalytic mechanism for MgB2/GG@Ru NFS was evaluated by trapping experiment of active species, verifying that superoxide (O2−) and electron (e−) contributed significant role in the dye degradation.

Overview of spark plasma synthesis and sintering of MgB2 superconductor

This study aims to investigate the sintering mechanisms of magnesium diborides using spark plasma sintering and optimize the sintering process conditions. Three different precursor materials were used: (i) commercial powder MgB2, (ii) mixtures of magnesium and nano boron powder (Mg + 2B), and (iii) mixtures of Mg and MgB4 powder for an ‘in-situ’ reactive synthesis. The density of the obtained bulks exceeds 90% of the theoretical density of the material. The structure and microstructure of the samples were analyzed using x-ray diffraction and scanning electron microscope, which were correlated with their superconducting properties. The critical current density (J c), pinning force (f p), and electrical resistivity (ρ) of the samples were also investigated. The results show that the in-situ reactive synthesis of MgB2 using magnesium and nano boron powder was the most effective method for producing high-quality samples with improved superconducting properties. The findings could lead to the development of more efficient methods for producing high-performance of MgB2 bulks.

Joining Superconducting MgB2 Parts by Spark Plasma Sintering: A New Technique with High Potential for Manufacturing Future Superconducting Devices

MgB2 pellets that were sintered by Spark Plasma Sintering (SPS) from commercial MgB2 powder were soldered by a SPS process and compared to unsoldered ones. The soldered samples were characterized by scanning and transmission electron microscopy which have shown no visible defects at the junction. SQUID magnetometry and four probes resistance measurements have confirmed that the soldered pellets present the same magnetic properties as the unsoldered ones. The presented soldering technique could find applications for manufacturing superconducting devices with custom shapes which could be used as rotors for motors and generators, magnets and magnetic shields. This new approach could generate a high potential interest for industrials in these and connected domains such as the development of new electrical planes using superconducting technologies.

Effect of grain size selection in ex-situ made MgB2 wires

Commercial MgB2 powder manufactured by Alfa Aesar was mechanically sieved in order to obtain powder samples with different particle size. As-sieved powders were used for single-core wires with FeNiCo sheath made by ex-situ process and heat treated at the same conditions, 950 °C/30 min. Resistive transitions of extracted MgB2 cores allowed to obtain the critical temperature and normal state resistivity for different particle size and estimate the connectivity by the Rowell approach. In field critical currents were also affected by the particle size of sieved MgB2 powder. The highest current densities were measured for the wire with particles 15-25 µm, which is explained by the dominant effect of grain-boundary pinning.

Commercial MgB2 superconducting wires at Sam Dong

Commercial MgB2 superconducting wires at Sam Dong

Oxidation, ignition and combustion behaviors of differently prepared boron-magnesium composites

Effect of magnesium on ignition and combustion characteristics of boron was explored for three materials containing ca. 23 wt% of magnesium. Boron was sintered with elemental Mg, leading to the formation of MgB2. This material showed the most homogeneous distribution of Mg. Boron was also mechanically milled with elemental Mg, as well as with commercial MgB2, respectively. The presence of Mg delayed the oxidation of boron in all materials. The delay was nearly the same for both milled powders and it was somewhat greater for the sintered material. The observed oxidation rate of all materials, however, was greater than for pure boron. All materials could be ignited by a heated wire at nearly the same temperature, although boron could not. Particles of all materials, including elemental boron, could be ignited by a CO2 laser with the lowest power required for the sintered material. The addition of Mg had a weak effect on particle burn times. Boron milled with elemental Mg showed distinct two-stage combustion patterns interpreted as delayed, and separate, Mg combustion. A small increase in the maximum pressure and rate of pressure rise is observed for the Mg-bearing powders in constant volume explosion experiments.

Design and characterisation of ex situ bulk MgB2 superconductors containing a nanoscale dispersion of artificial pinning centres

MgB2 pellets containing a nanoscale dispersion of artificial pinning centres have been successfully manufactured through a powder metallurgy route based on the oxide dispersion strengthened (ODS) concept more usually used for steels and superalloys. Commercial MgB2 powder and Y2O3 nano-powder were mechanically alloyed in a high energy planetary ball mill and consolidated using the field assisted sintering technique. The composite powders were ball milled for different times up to 12 h and characterised by means of particle size analysis, x-ray diffraction (XRD) and scanning transmission electron microscopy (STEM). The microstructure and superconducting properties were characterised by density, XRD, STEM and magnetic property measurements. The powder microstructure comprised Y2O3 particles dissolved into the MgB2 matrix. After consolidation there was a near-uniform dispersion of precipitated YB4 and MgO particles. A bulk 0.5 wt% Y2O3-MgB2 composite showed the best superconducting performance with a significant improvement in J c at high field compared with unmodified MgB2, and only a small reduction in T c . The results suggest that the ODS concept is promising to improve the superconducting properties of MgB2.

Enhancement of critical current density in MgB2 bulks burying sintered with commercial MgB2 powder

Pellets of Mg and B mixture was buried by commercial MgB2 and one-step sintered at 800 °C, two-step sintered at 750 °C followed by 900 °C, and two-step sintered at 800 and 600 °C, respectively. Although an increasing amount of MgO was found in the burying sintered samples, the particles were considerably refined and embedded in the MgB2 grains, as the MgO is likely to be from the absorbed O2 on the commercial MgB2. Commercial MgB2 served as nucleation sites for newly formed MgB2 grains, which mostly generated at solid–solid reaction stage following Ostwald ripening mechanism. Apart from low crystallinity, such low-temperature synthesis of MgB2 induced defects including grain boundaries and second-phase particles as effective pinning centers. As a result, the critical current density is enhanced at high field in the burying sintered samples, in contrast with the one without burying.

Dry cryomagnetic system with MgB2 coil

MgB2 may be the future superconducting wire material for industrial magnets due to it’s higher operation temperature and potentially lower cost than low temperature superconductors (LTS) have. We designed a compact cryomagnetic system with the use of MgB2. The possibility of creating a magnet with a central field of 5 T from a commercial MgB2 wire by the “react and wound” method was investigated. The magnetic system is cooled by a cryocooler through a copper bus. The magnet has a warm bore diameter of 4 cm. The design of a magnet consisting of three concentric solenoids is proposed: an internal one of high-temperature superconductor (HTS), an average of MgB2, and an external of NbTi. The operating current of the system is 100 A. Two pairs of current leads are used. A separate pair of current leads for power supplying NbTi coil allows testing of MgB2 and HTS coils in an external field. The load curves for each of the magnets are calculated.

Critical currents of Rutherford MgB2 cables compacted by two-axial rolling

Two types of Rutherford cables made of two strand layers of commercial MgB2 wires manufactured by Hyper Tech Research, Inc. have been made. Flat rectangular cables consisting of 12 single-core MgB2/Nb/Cu10Ni, or 6-filaments MgB2/Nb/Cu strands, both of diameter 390 mewm, were assembled using a back-twist cabling machine with transposition length of 20 mm. In order to analyze impact of the cable compaction on critical currents, cables were two-axially rolled, each by a single step reduction of 3.5%−29.7% to thickness range of 0.775−0.62 mm. It was found that by increasing the packing factor (PF) of cable above 0.79, the critical current begins to increase. It is improved nearly two times up to the PF limit 0.89. Compaction over the PF limit introduced cable degradation and decrease of critical current. Bending tests applied to cables showed that critical current degradation starts below the bending diameter 120 mm for 6-filaments Cu sheath and 70 mm for single-core Cu10Ni sheath cable. Tensile tests showed similar irreversible strain values for the both types of cables. Rutherford cables assembled of single-core strands are promising for low field (2.7−4 T) applications where low bending diameters are required.

Effect of additions of Ca compounds to the filling powder on the reduction of MgO and the critical current density properties of ex situ processed MgB2 tapes

We have studied the effect of additions of Ca compounds, CaC2 and CaH2, to the filling powder on the transport critical current density (Jc) properties of ex situ processed MgB2 tapes. These additives are expected to reduce MgO present in as-received commercial MgB2 powder and improve the grain coupling of MgB2 in the tapes. Heat treatment of the hand mixed powders, MgB2 and the Ca compounds, brings about both disappearance of MgB4 and formation of CaB6 and Mg, compared with heat treated MgB2 powder. They are observed much more clearly for the mixtures with CaH2 addition. The MgO content decreases by the additions of those compounds. Neither of the a-axis and the c-axis parameters of MgB2 are changed by the additions, suggesting no reactions of substitutions of Ca for Mg and C for B in MgB2. The Jc properties of the tape samples using the hand mixed filling powder with CaC2 addition degrade, whereas those with CaH2 addition are slightly improved. When using the ball milled mixtures, slight contraction of the a-axis parameter occurs only by CaC2 addition. Although the additions cause the reactions similar to those observed for the hand mixed powders, the MgO content reduces more clearly, compared with the MgB2 content. The Jc properties of the tapes are enhanced in the high-field and low-field regions by the additions of CaC2 and CaH2, respectively. These enhancements are attributed to the carbon substitution, the improved grain coupling and the increased content of MgB2.

Thermal conductivity and stability of commercial MgB2 conductors

This paper presents a study of the thermal transport properties of MgB2 tapes differing in architecture, stabilization and constituent materials. The temperature and field dependence of thermal conductivity, was investigated both along the conductor and in the direction perpendicular to the tape. These data provide fundamental input parameters to describe the 3D heat diffusion process in a winding. Thermal transport properties—even in field—are typically deduced using semi-empirical formulas based on the residual resistivity ratio of the stabilizer measured in absence of magnetic field. The accuracy of these procedures was evaluated comparing the calculated κ values with the measured ones. Based on the experimental thermal conduction properties and critical current surface we determined the dependence of minimum quench energy and normal zone propagation velocity on the operating parameters of the conductor. The correlation between thermal properties and tape layout allowed us to provide information on how to optimize the thermal stability of MgB2 conductors.

Development in processing of MgB2 cryo-magnet superconductors

Spark Plasma Sintering is a promising rapid consolidation technique that allows a better understanding and manipulating of sintering kinetics and therefore makes it possible to obtain MgB2-based ceramics with tailored microstructures. Commercial MgB2 powders were spark plasma sintered with an applied mechanical pressure, leading to MgB2 pellets with 99% relative density. The obtained samples show a sharp superconducting transition with an onset at 38.5 K. The critical current density was measured equal to 5.6 × 104 A/cm2 at 20 K under 1 T applied field. Otherwise, the trapped field was also investigated. A 5 mm thick pellet with a 20 mm diameter prepared with optimized processing temperature demonstrated a trapped field of 1.2 T at 15 K and up to 2.5 T at 10 K.

Effect of the Powder Strain State on the Mechanical Properties of MgB$_{2}$ Tapes

Starting from commercial MgB2 powder of 325 mesh, we further reduced the agglomerates by attrition milling in nitrogen atmosphere to minimize oxidation. Using different milling media (tungsten carbide, yttria stabilized zirconium oxide and stainless steel balls), we obtained different milling energies and, consequently, powders with different strain states. Crystallite size and micro-strain within the grains were estimated by x-ray diffraction. We investigated the changes in the superconducting properties with the use of different milling media by measuring the magnetic moment as a function of temperature in a SQUID magnetometer, after zero field cooling. Single-filament stainless steel-sheathed wires and tapes were prepared with the obtained powders. The flexural properties of the tapes were determined by 4-point bending micro-tests. The use of a precursor powder with a higher strain resulted in more flexible wires and tapes, with lower flexural strength. Three different regions were determined for the maximum normal stress-strain curves. Some in-situ monel-sheathed hollow wires and ex-situ titanium-sheathed tapes and monel-sheathed 7-filament tapes were measured for comparative purposes. We discuss the correlation between sheath yielding and superconducting core cracking with the possible critical current degradation due to mechanical failure of the wires and tapes.

ChemInform Abstract: Synthesis of Amorphous Mg(BH4)2 from MgB2 and H2 at Room Temperature.

AbstractUnsolvated amorphous Mg(BH4)2 is prepared by reactive ball milling of commercial MgB2 under 100 bar hydrogen pressure for up to 100 h with a yield of about 50%.

Intensive Milling Effect on the Properties of ${\rm MgB}_{2}$ Tapes

Starting from commercial MgB2 325 mesh powder, we further reduced the agglomerates by attrition milling in nitrogen atmosphere to minimize oxidation. The milling time was varied between 10 and 40 hours, using two different types of milling media: yttria stabilized zirconia and CW balls. We studied the influence of milling time and milling media on the broadening of x-ray diffraction peaks and the superconducting properties of the powders. A highly energetic milling markedly decreased the crystallite size but damaged the superconducting properties. We observed a decrease of the Tc, consistent with the observed crystallinity loss. We prepared tapes with the 40-hours-milled powders and analyzed the resulting microstructures when using a soft (silver) and a hard (stainless steel) metal sheath, heat treated for 1 hour at 615degC in argon. This temperature was chosen after observing the existing reactions between magnesium diboride and silver by DTA analysis of the powders, so as to minimize the reaction at the superconductor-silver interface. We characterized the superconducting properties by determining the magnetic Jc in a SQUID. We performed 4-point bending micro-tests on the tapes to evaluate their mechanical strength.

Winding and Testing of Large Bore Solenoids, and Study of Quench Propagation in Short Coils Made With Multifilament ${\rm MgB}_{2}$ Tape

Magnesium diboride has a great potential for magnet applications in many areas such as high energy physics, MRI, and space-related projects. The main goal of our ongoing research effort is to study magnet design, winding process, and magnet testing. Last year, in a 14 cm bore solenoid we reached a central magnetic flux density of 1 tesla at 16 K with almost 500 m of commercial multifilament tape. In order to design appropriate quench detection and protection systems, it is important to define the quench propagation characteristics in solenoids made with this superconductor. In this work we show our experimental activity, from winding to testing, with a 14 cm bore solenoid made with commercial multifilament MgB2 tape. In particular we address a quench propagation study on a coil made with 2 m of the same tape.

Doped MgB2 prepared by field assisted sintering technique

Field-assisted-sintering technique (FAST) has been employed to obtain polycrystalline MgB2 samples, pristine and doped with 5 mol % SiC and B4C, using commercial MgB2 powder. The mass density of the samples is around 90% of the theoretical value. The response in magnetic field shows that the upper critical field and the irreversibility field are depressed by comparison with pristine MgB2. The latter has an upper critical field Hc2=14.37 T close to the values reported for the single crystals. Scaling analysis of the field dependence of the pinning force suggests that both addition of SiC and B4C stabilizes two well defined pinning regimes in FAST-processed MgB2. At low temperatures, T< 24 K (H≤5T), the pinning occurs at the grain boundaries whereas at higher temperatures there is a mixed pinning. These effects are supposed to be the combined result of relatively low level of C diffusion into the lattice of MgB2, specific features in formation of the grain boundaries during FAST processing and the influence of the added materials on morphology. At high fields (above 8.5T) and 4.2K, doped samples are superior to the pristine one from the critical current density viewpoint.

Structure and superconducting properties of pure and variously doped bulk MgB2 obtained by uniaxial and isostatic hot pressing

The preparation of dense MgB2 bodies, undoped and doped with different atomic species (Na, Ag, Y), was performed via reactive sintering by uniaxial and isostatic hot pressing, starting from the pure elements, and compared with undoped samples obtained by commercial MgB2 powder. The superconducting characteristics of the obtained materials, namely critical temperature (Tc) and current (Jc), were obtained through ac susceptibility measurements and compared to their structural features, like phase purity and secondary phases formation and distribution in the MgB2 matrix. Both the adopted hot pressing techniques gave rise to undoped MgB2 pieces exhibiting phase purity in the range 85–95% and relative density above 80%; although in most cases the doped samples underwent higher extents of phase decomposition and lesser densification, they all exhibited higher critical temperature and current compared to the corresponding undoped material, indicating a net influence of the doping on the superconducting behaviour of MgB2, particularly Ag and Y. An opportune quality factor was adopted, to obtain a more reliable comparison between the different MgB2 samples and evaluation of the samples goodness, in relation to their superconducting characteristics. It was put in evidence that low amounts of doping can improve the superconducting behaviour of MgB2 and that this influence can be addressed in terms of pinning centres, as there was no experimental evidence of an actual atomic substitutions in the MgB2 crystal.

Complete flux jump in bulk MgB2 sintered under high pressure

Sintered MgB2 system was prepared under 3.5GPa in the WC cubic anvil cell above 1100°C for 1h from the commercial MgB2. In this system, a few large flux jumps (FJ) were observed in the magnetization curve at 5K on the field increasing process. Especially, the complete FJ occurred near 0 field. The number of large FJ decreased at higher temperature of 20K. We estimated the flux density difference between the center and surface of the sample from the extended critical state model and magnetization data. The generation condition of FJ was discussed using Schwartz–Bean’s theory and Wipf’s theory. The FJ was suppressed at 20K by the MgCu2 doping. We assumed that the present large FJ’s were induced by the enlarged sample thickness by high pressure sintering. A few mol% of MgCu2 impurities reduced the thickness of MgB2 phase or enhanced the thermal conductivity of HP sample.